This invention is generally concerned with the purification of liquids. Of particular interest is the treatment of water which contains unacceptable amounts of dissolved salts, such as seawater, brackish water or hard water. Such waters may be purified by forcing the water through a semipermeable reverse osmosis membrane, leaving behind the contaminants or salts which do not pass through the membrane. This method may be used for softening of hard waters, but heretofore the pressures required to operate the separation process made use of reverse osmosis for such a purpose less attractive.
A reverse osmosis membrane must reject a high fraction of the dissolved salts. In addition, if chlorine is added to water as a disinfectant, the membrane generally must be tolerant to the chlorine. It is particularly important that such membranes pass a relatively large amount of water (i.e., have a high flux) through the membrane at relatively low pressures.
Reverse osmosis membranes have been made from a wide variety of known polymeric materials. While many of these polymeric materials can reject a large fraction of the salt, some cannot provide a sufficiently high flux of water.
Many U.S. patents describe various membranes which are useful in desalination processes, see for example, those cited and discussed in U.S. Pat. No. 4,830,885. One of the earliest patents to describe membranes of the type used in the present invention is U.S. Pat. No. 3,744,642 to Scala et al.
The semipermeable membrane used in the desalination process ordinarily will be relatively thin in order to increase the flux. Thus the membrane often is formed on a porous support. Scala et al. suggest reacting a broad group of amines or bisphenols with acyl halides or sulfonyl halides on a support material to form thin membranes. This provides strength to the composite. The supports should possess pore sizes which are sufficiently large so that the water (permeate) can pass through the support without reducing the flux of the entire composite. Conversely, the pore size should not be so large that the thin semipermeable membrane will be unable to bridge the pores or will tend to fill up or penetrate too far into the pores. Scala et al. suggest that above about 8 microns the rejection of impurities is reduced.
U.S. Pat. No. 4,277,344 discloses a reverse osmosis membrane made in situ according to Scala et al., which has been prepared from a polyacyl halide and an arylene polyamine. According to the '344 patent, no advantage was found for surfactant and acid-accepting additives and it is preferred to carry out the interfacial polymerization without the presence of acid acceptors. The '344 patent teaches that the membrane contains a plurality of sites having the formula:
Ar(CONH--).sub. 2COOH
in which Ar represents the aromatic nucleus residue of the polyfunctional aryl halide. It is of interest with respect to the present invention that according to the '344 patent, solvents for the polyacyl halides that dissolve or plasticize the support material should not be used. In accord is U.S. Pat. No. 4,619,767 to Kamiyama et al. which states that it is necessary to avoid solvents for the crosslinking agents (e.g. acid halides) which dissolve or swell the porous substrate.
It has been generally believed that if solvents for the support material are used, the surface of the support is disrupted and it is difficult, if not impossible, to produce a uniform desalinizing layer. Also, it has been generally believed that use of a solvent for the support will obscure the pore structure of the support, increasing the resistance to water flow through the support, and thus reducing the flux. Research sponsored by the U.S. Dept. of the Interior on seawater desalination and described in the reports by the contracting organizations also support this view of the prevailing opinion of those skilled in the art.
In U.S. Patent No. 4,830,885 an improved supported membrane is disclosed in which a polyhydric compound (which does not dissolve typical support materials), is included with the amine solution in preparation of the membrane. The polyhydric compound provides improved flux through the membrane while maintaining the high rejection of the dissolved salts.
The present invention provides still further improvement in the performance of supported membranes by using polar aprotic solvents which are capable of dissolving or plasticizing the support material.